The connectors that attach cables to high frequency test equipment, such as oscilloscopes, have a significant effect on the fidelity, and even the very ability, of the connection to convey the signal of interest. This arises from the customary practice of using an actual transmission line with a controlled characteristic impedance (Z0), which is often 50 Ω, as the connecting medium between the test equipment and the work piece. As is well known, the characteristic impedance of a transmission line is affected by its mechanical dimensions, and the presence of a connector between two sections of a coaxial transmission line is an opportunity for unwanted changes in Z0 (discontinuities) that cause reflections and impair the fidelity of the transmitted signal. Not all high frequency connectors are created equal, and it is often the case that the more conveniently manipulated connectors (e.g., BNC) are ill suited for high frequency service (standard BNC begins to ‘leak’ above 500 MHz, and is extremely visible as a significant discontinuity on a TDR—a Time Domain Reflectometer).
Connectors that offer suitable performance have been variously developed and adopted as commercially available ‘standard’ items. As service at ever higher frequencies is contemplated the physical size of the coaxial transmission line becomes an issue. While the characteristic impedance of a coaxial transmission line is determined by a ratio of diameters and an intervening dielectric constant, when the particular dimension of the diameter difference becomes a significant fraction of a wavelength the physical structure of the resulting transmission line can (simultaneously) support different modes of propagation (which is called ‘moding’). These different modes (physical orientations of the electric and magnetic fields relative to the conductors of the transmission line) do not all behave the same way as they propagate, but they can interact, and the resulting mischief can be likened to an internal ‘trafficjam’ for the signal within the cable. Thus it is that the tendency is to employ coaxial structures of smaller diameter as bandwidth increases (to prevent moding), despite the fact that small cables are usually more lossy than their larger counterparts.
Smaller cables mean smaller connectors, which in turn are more delicate and more easily damaged than their larger counterparts. And while the larger 7 mm line of connectors (GR-874, N, APC-7) are comparable in size to the adult human finger and thumb (allowing for comfortable manipulation) and are generally usable up to 18 GHz, the trend for high frequency oscilloscopes has been to move toward the use of 3.5 mm connectors (APC-3.5, SMA) that are good up to 26 GHz, and can be foreseen to use the 2.4 mm connector. To achieve their promised performance these connectors need to be properly mated, which includes proper tightening of a small threaded nut on the male portion of the connector that draws the two connector halves together. This operation is generally regarded as being noticeably fussier than for a 7 mm counterpart.
Another issue concerning high frequency oscilloscope probes is that they are generally active probes, meaning that there is an actual powered amplifier at their ‘business end’ near where the probe tip is, and that not only must the front panel connection at the 'scope accommodate a 3.5 mm connector for a transmission line, but also various conductors for power, ground, and ‘house-keeping’ information must be dealt with. (Housekeeping information can include such things as reporting by the probe of its model number or type, and calibration information specific to that particular probe.)
Thus we arrive at the subject matter of the incorporated U.S. Pat. No. 6,884,099 B1. It deals with various aspects of a ‘push-on’ coaxial connector and assorted auxiliary (house keeping) connections that are housed in a probe pod. The probe pod is attached to the front panel, and all of its connections, both coaxial and auxiliary, are made by simply first aligning the pod to the front panel and then pushing the pod against the panel, followed by a short tightening movement of a locking tab (or lever) by the thumb. The probe pod is removed by an opposite motion of the thumb against the locking lever while pulling the probe pod away from the front panel. The incorporated U.S. Pat. No. 6,884,099 B1 deals with the use of a modified (precision) BNC connector in such a push-on probe pod. The BNC design is well suited for such a use, as the bayonet latching mechanism involves a quarter turn that can be accommodated by spring loading the BNC latch, while a locking feature is coupled to the motion imparted to the locking lever by the thumb. The housekeeping connections are made with spring-loaded ‘pogo’ style pins in the probe pod that press against corresponding pads on the front panel.
When considering how to extend the frequency range of the push-lock connector for a probe pod there is a choice of whether to start from scratch and design an entirely new connector, or, as with the modified precision BNC connector, to begin with an established design and adapt it to operate in the new environment. The interchangeable SMA/APC-3.5 connector specifications afford an attractive starting point. However, they rely upon several turns of a threaded nut to draw the mating parts together. We would like to have something that, as far as the operator is concerned, works or is used in the same manner as does the push lock BNC connector of probe pod of U.S. Pat. No. 6,884,099 B1, but which has the electrical performance of the APC-3.5 connector. It seems we need to combine the convenience of a BNC-style latch with the performance of 3.5 mm innards. What to do?